1. Field of the Invention
The present invention belongs to the technical field of an optical pickup for a disc player for recording information on a disc and reproducing the information from the disc, and particularly relates to a magnet to be used in an actuator of an objective lens.
2. Description of Related Art
An optical pickup device used for recording information on an optical disc or reproducing the information therefrom generally includes an objective lens and an actuator (lens driving device) for driving the objective lens. The actuator moves the objective lens in a focus direction and a tracking direction to correct the relative positional relationship of the objective lens with respect to the information recording surface of the disc and the track formed on the information recording surface. That is, the actuator adjusts the distance between the objective lens and the information recording surface of the disc based on a focus error signal so that a light spot of a predetermined size is formed on the information recording surface of the disc. Moreover, the actuator adjusts the position of the objective lens in the disc radial direction based on a tracking error signal so that a light spot is positioned correctly on the track of the information recording surface of the disc.
According to the known actuator, the position control of the objective lens in the focus direction and the tracking direction are executed by the electromagnetic force generated by the control current flowing in a coil formed on a printed circuit board and a magnet disposed adjacent to the coil.
A coil substrate provided with a focus coil and two tracking coils is mounted on a lens holder for supporting an objective lens, and such a lens holder is disposed within the magnetic field generated by a magnet. When the control current flows through the focus coil and the tracking coils, a driving force of the lens holder is generated and the lens holder is moved in the focus direction and the tracking direction. By controlling the polarity and the amount of the control current applied to the focus coil and the tracking coils, the objective lens can be disposed at the right position with respect to the information recording surface and the information recording track of an optical disc.
The magnet is magnetized, in advance, to a predetermined pattern by a magnetizing device. For example, in the case of a conventional actuator, in order to provide an appropriate magnetic field to a focus coil and two tracking coils, the magnet is magnetized using a magnetizing core with a “U”-shaped pattern.
However, in the case where the magnet is magnetized using the magnetizing core of a “U”-shaped pattern as mentioned above, since decrease of the magnetic flux density cannot be avoided at the position of the coil substrate position, disposed adjacent to the magnet, corresponding to the corner part of the “U”-shaped magnetizing core, the magnetic field cannot be generated in an ideal pattern at the coil substrate position. Therefore, there is a such problem that the sensitivity of the actuator is deteriorated or an adverse effect is given to the rolling characteristic.